Associated Press

AUSTIN, Texas (AP) — The hunt for the serial bomber who has been leaving deadly explosives in packages on Austin doorsteps took a new, more sinister turn Monday when investigators said the fourth and latest blast was triggered along a street by a nearly invisible tripwire.Police and federal agents said that suggests a "higher level of sophistication" than they have seen before, and means the carnage is now random, rather than targeted at someone in particular. Underscoring that point, a relative says the most-recent explosion left what appeared to be nails stuck in his grandson's knees.